The term "SPA" wasn't in use until at least after 2009 and gmail was probably using some hacky AJAX (XMLHTTPRequest wasn't even standardized until, what, 2006?). Chrome wasn't launched until 2008 so they weren't able to get away with just adding the APIs they needed into the browser. Backbone wasn't even released until 2010 and Angular probably wasn't conceived of internally until late 2008.
Yes, gmail might have had some SPA-like behaviour in 2004-2006 but it was nothing like what we have today. Pretty sure I got access in 2005 because I knew someone who worked at google, and it was mostly doing full refreshes between actions at the time, like pretty much the entire rest of the web
SPA is just an abbreviation of "single page application" and only means a web app that doesn't do full page reloads, it doesn't require the use of any specific framework, so Gmail definitely qualifies and it used it from version one. It wasn't even the first, XMLHttpRequest was created by Microsoft for Outlook Web Access and it shipped in 1999 in IE5. Before Gmail there was also Oddpost, another SPA webmail app, so people started using it to build non-reloading web apps almost immediately. Gmail was the iPhone of web mail: not the first, no real new tech, but just very well done overall and popularized the concept.
You seem to be trying to redefine SPA to mean something else and much vaguer - the use of some specific frameworks, or not being "hacky" whatever that means - but your history here is just off.
Also, jquery was written in 2005 and launched in 2006, and became popular very fast. It was definitely pretty well known by 2008 and of course jquery itself was nothing new, most companies had very similar sets of utility libraries for JS that they used. Jquery just happened to be open source and have a monomaniacal focus on terseness at almost any cost.
Reality is the web has changed relatively little since 2008. It got some more APIs that weren't new, they were used in native desktop/mobile apps for many years beforehand, and that's about it.
Yes, gmail might have had some SPA-like behaviour in 2004-2006 but it was nothing like what we have today. Pretty sure I got access in 2005 because I knew someone who worked at google, and it was mostly doing full refreshes between actions at the time, like pretty much the entire rest of the web